Add support for cython, depend on sagemath, instructions for PyPI

[mmasdeu]

After using the template to distribute my own project, I have added some functionality to the package. In particular:

1. Added a cython file and the necessary changes to make it work.
2. This being a Sage package, it depends on the python package sagemath. This is a tiny python package available at PyPI that allows to check which version of Sage is installed.
3. Added some basic instructions to distribute the code to PyPI.

[videlec]

You can get the list of tarballs without using Python in one line

wget $SAGE_SERVER --quiet --output-document=- | grep "sage-.[^\s]*bz2" --only-matching

bash is much better than Python for that kind of things

[videlec]

Would be much cleaner to set a variable

SAGE="$HOME/SageMath/sage"

and use it everywhere sage is called

[seblabbe]

I am currently at Sage Days 117. There was a demo earlier this week on how to share code and create a package where we used sage_sample. I am now taking a look at all the PR that have been here for years now...

The current PR has conflicts. I let the author or someone else fix them.